Nowadays preventive maintenance of a machinery is performed on occasions based on the size of a service agreement between a service department responsible for service of the machinery and a customer using the machinery. The amount of inspections per year can be settled in a service agreement. During inspections, one or several field service engineers are visiting the customer. The field service engineers perform preventive maintenance activities such as lubricating, cleaning, replacing packings, oil changing, etc. Normally, customers demand indications of a coming failure in order to give the field service engineer permission to replace parts. In many cases, service of the machinery, such as the service of an industrial robot, is performed in a reactive manner, which means that the robot is run until a failure occurs.
In this specification, the machinery referred to is exemplified by particularly pointing out maintenance of industrial robots.
Industrial robots are used in a variety of applications and environments and the usage of a robot in terms of load and motion parameters varies to a great extent from robot to robot. Therefore, usage of a static maintenance schedule for all robots in a population will cause many robots to have parts exchanged too soon, while other robots will have parts exchanged too late with undesired and costly production stops as a result.
industrial robots when sold to a customer are delivered together with product manuals. In the product manual there are recommendations regarding required maintenance activities and replacements. Times for the recommended replacements of exposed parts are not based on the usage of the industrial robot, whereby overloaded parts of industrial robots might break down before the recommended replacement times.
Machinery, such as industrial robots and comparative technical systems are developed to manage more and more functions and the complexity of such technical systems gets increasingly higher. More competition and higher demands on markets has forced companies to produce high qualitative products using more efficient production. In order to obtain the more efficient production, companies require increased reliability of the technical systems. Therefore maintenance of the machinery has drawn more attention and has become a very important competition weapon.
Industrial companies demand products with high availability and high reliability of operation. Other than investing in even more high quality technical systems, the users of a certain machinery, such as an industrial robot, set aside more and more resources to maintenance of the machinery. The industrial companies do accept production stops but they want to know in advance when the stops will occur.
An example of a way to avoid unnecessary stops of an industrial robot utilizing preventive maintenance is disclosed in U.S. Pat. No. 5,587,635. Said preventive maintenance relies on monitoring a specific component by means of direct sensing certain properties of robot, whereby the remaining lifetime could be estimated for the component or a drive system.
Another document describing prior art is EP 1 162 524 A. In this document the wear of one unit of, for example, a robot part is supervised and measured. The lifetime of the unit is directly associated with a level of wear. When said level is reached the unit is repaired or exchanged. There is no connection between factors having impact on the lifetime, but for speed and force, i.e. pure mechanical wear.
Still another document: Jardine et al, “a Review on machinery diagnostics and prognostics implementing condition-based maintenance”, Mechanical systems and signal processing, London, GB, vol. 20, no. 7, October 2006, pages 1483-1510, XP005507568, ISSN: 0888-3270, is directed to condition based maintenance. The idea in this document is based on data being collected from a specific unit, such as a part of a machinery. Said data are processed in a computer. The condition of the unit is supervised and when the state of the unit in some way is deteriorated, a supervising device alerts that something is wrong with the unit. Nothing is stated in this document about the relevance of, e.g. the impact of external factors.
One more document referring to maintenance of a unit of a machinery is: Xiang Li, Junhong Zhou, Hao Zeng, “A intelligent predictive engine for milling machine prognostic monitoring”, IEEE, International conference on industrial informatics, August 2006, XP002442651, whole document. In this document influencing factors are monitored, wherein signals representing the influencing factors representing measured values are monitored. Multiple regression is performed on said monitored signals, whereupon a lifetime of the monitored unit is estimated.
Document WO 2005/109133 A describes an invention, wherein a computer supervises and determines a point of time for maintenance of a monitored unit of a machinery.
The documents referred to above describe merely monitoring by use of signals collected from sensors measuring the state of the supervised unit in relation to mechanical impacts of the lifetime of the unit and they do not consider the impact of environmental factors that can have influence of the lifetime of a specific unit or of a complete machinery related to its specific use. These factors can be, e.g. temperature, humidity, range of movements in space, linear speed, rotational speed, accelerations. Further, neither do said documents consider data based on collected statistical data for lifetimes of a population of said specific unit of the machinery, neither times for preventive replacements performed for the specific part.